Watering down the syllabus
....meanwhile the A-Level syllabus (especially Maths) gets further watered down, as the entrants are poorer in the subject coming in at 16. Uni Porfessors moan permanently at the remedial standard of maths in new undergrads
If double science is a watered down science, and all subjects are becoming more watered down (as evidenced by maths capability above) then it stands to reason that double science at GCSE is providing a far poorer preparation for A-Levels then single separate sciences.
They offered double maths at my school (20 years ago), back then it was mandatory for the thickie kids who elected not to take single sciences. There was a bar on progression to A-level sciences from double-sceince GCSE.